Electron discharge device cathode and method of making same



May 8, 1956 R. w. NEWMAN 2,744,838 ELECTRON DISCHARGE DEVICE CATHODE ANDMETHOD OF MAKING SAME Filed Nov. 5, 1951 ROUGH NICKEL -AN 0 COBALTPARTIALLY ALLOYED "3 \ALKALINE EARTH METAL OXIDE Inventor": RQber-tW.Newman,

b Z W3 HisAtLm United States Patent O ELECTRON DISCHARGE DEVICE CATHODEAND METHOD OF MAKING SAME Robert W. Newman, South Schenectady, N. Y.,assignor tYo (lieneral Electric Company, a corporation of NewApplication November 3, 1951, Serial No. 254,704 15 Claims. (Cl.117-217) My invention relates in general to electron discharge devicecathodes and processes of making same. My invention relates inparticular to nickel cathodes which are inhibited against flaking orpeeling of the electron emissive substance applied to the surfacesthereof.

In the use of electron discharge devices, one of the commonestlimitations to the useful life thereof is a reduction of the electronemissive capabilities of the cathodes therein which is occasioned by theflaking or peeling of the surface coating of emissive oxide materialfrom the surface of the cathodes. This problem is especially accented indischarge devices employing nickel cathodes in a mercury vaporatmosphere, presumably because of an amalgamating or wetting tendency ofmercury with nickel whereby mercury destroys the bond at the interfacesbetween the nickel cathodes and the emissive oxide layers thereon. It,therefore, follows that reduction of this flaking or peeling results inan improved cathode impart-' ing longer useful life to electrondischarge devices.

It is a primary object of my invention to provide a new and improvedcathode for use in electron discharge devices.

It is a further object of my invention to-provide a It is a furtherobject of my invention to provide a process of cathode treatment toproduce the new and im proved cathode of my invention.

And it is a still further object of my invention to provide such animproved cathode from which flaking of an emissive oxide coating in -amercury vapor atmosphere is particularly greatly reduced.

Briefly stated, the cathode of my invention comprises a cathode basemember which may have any physical shape and which includes a surfaceportion thereof containing at least 3 percent cobalt and the remainingpercentage, by weight, substantially pure, i. e., commercially pure,nickel. A suit-able electron emissive oxide layer, for example, an oxideof one or more alkali earth metal elements, is coated on the surfaceportion to emit electrons when raised to an elevated temperature. I havefound that such a surface portion having at least 3 percent cobalt innickel provides a mossy, dendritic base upon which the emissive oxidecoating is held, small tooth-like dendrites projecting from the surfaceportion to anchor the oxide coating against flaking thereof otherwisecaused by mechanical and thermal shocks. The cobalt in the surfaceportion of the cathode further provides a rougher more porous metallicbase upon which the emissive oxide is coated, therefore providing a basewhich has little tendency to wet or amalgamate with mercury in thepresence of mercury vapor, as does nickel, so that the bond between .theoxide coating and the surface portion of the cathode is not weakened bythe action of mercury.

The cathode of my invention may be of either the reotly or indirectlyheated type and may comprise a base member which is composed of acommercial grade of nickel, as is commonly employed for cathodes. Inaccordance with my invention, the surface of the nickel base for thecathode is processed to inhibit flaking or peeling of a later appliedemissive oxide coating there'- from. This inhibiting treatment comprisesa powdered mixtureof cobalt and nickel compounds suspended in a bindervaporizable by heat, and a solvent for the binder. The cobalt and nickelcompounds are selected from those which are reducible by heat tometallic cobalt and nickel respectively, for example, oxide, carbonate,or hydro)? ide compounds of cobalt and nickel. The second step is toheat the cathode and applied powder to a sufficient temperature in amanner to vaporize the binder, reduce. the cobalt and nickel compoundsto metallic cobalt and nickel respectively, and to at least partially'alloy the cobalt and nickel thus formed. The resulting surface portionof the cathode, which contains an appreciable amount of cobalt at leastpartially alloyed with nickel, is darker than a nickel surface and has amossy, den-dritic structure. Electron emissive oxide compounds are thenting or amalgamating with mercury. As a result, theemi-ssive oxidecoating is maintained on the cathode withmuch more stability, even inmercury atmospheres. While I have found it possible to make the entirecathode member, including a surface portion thereof, of a cobalt-nickelalloy to successfully inhibit oxide coating flaking, the extremely highcost and difliculty in procuring adequate amounts of cobalt for thispractice renders it more practical -to employ the inhibiting processdescribed whereby only a small amount of cobalt is required in thesurface port-ion of the cathode base member. In practice, as little as 3percent, by weight, of cobalt in the surface portion of the cathode basemember definitely inhibits oxide coating flaking. Accordingly, in theprocess described, the mixture of the cobalt and nickel compounds may beso proportioned that, when reduced, the resultant deposit is at least 3percent cobal and the rest substantially nickel.

The novel features of my invention are pointed out with particularity inthe appended claims. However, for a better understanding of theinvention, together with further objects and advantages thereof,reference should be had to the following description taken inconjunction with the accompanying drawing, wherein:

Fig. l is an enlarged elevational View of a cathode embodying thefeatures of my invention, the cathode being only partially shown andpartially broken away for clarity; and Fig. 2 is a side elevational viewtaken of Fig. 1.

Referring now to the drawing, I have illustrated for simplicity andconvenience only a strip portion of a cathode which embodies thefeatures of my invention and known in the art, alkaline earth metaloxides serveas an emitter of electrons when elevated to a sufficiently Jembodying my invention may have any desired physical shape and may beeither of the directly or indirectly heated type. It will be understoodthat the strip portion of a cathode I have illustrated in the drawingmay be a part of a ribbon or wire cathode which is directly heated bythe passage of current therethrough, such as shown and described in U.S. Patent 1,895,858, issued January 31, 1933, in the name of W. G.Morgan. Further, the illustrated strip portion may be a part of a hollowtubular cathode within which a heating filament is positioned to radiateheat to the cathode itself. Such an indirectly heated tubular cathode isshown, for example, in the patent of M. Bariess et al., #1,855,901,issued April 26, 1932.

There has long been a problem in the art of electron discharge devicesoccasioned by the flaking or peeling of this emissive surface coatingwhen applied directly to the nickel base because the nickel base at theinterface between the nickel and the emissive coating is smooth and thebond is not appreciably strong. Mechanical or thermal shock may causeall or part of the emissive coating to be flaked or peeled off thenickel base, thereby considerably reducing the emissive capabilities ofthe cathode and impairing useful operation of the discharge device ofwhich it is a part. This problem is particularly significant with regardto cathodes employed in devices having mercury vapor therein, becauseduring storage or non-operating times of the device when the cathode iscool, mercury condenses on the cathode and penetrates small openings inthe emissive coating to amalgamate with or form a mercury monolayer onthe nickel base. Mercury at the interface between the nickel and theemissive coating spreads and destroys the bond between the nickel andthe coating and it has been observed that in periods of less than onemonth substantially all of the coating is flaked off and useful emissionfrom the cathode precluded.

A cathode embodying my invention, as described, is free from thisflaking or peeling action of the emissive coating. This results fromseveral important effects produced at the interface between the emissivecoating 3 and the surface portion 2 containing cobalt and nickel. First,the surface portion 2 is at least partially a nickel-cobalt alloy whichis fused and alloyed into the nickel base member 1 because ofcharacteristic similarities of cobalt and nickel, so that the surfaceportion 2 is strongly bonded to the base member 1. Second, the surfaceportion 2 consists of a dark, mossy, dendritic composition withdendrites projecting therefrom so that when the emissive coating 3 isapplied, the dendrites stick into, and some entirely through, thecoating 3, thereby firmly anchoring the. coating against mechanical andthermal shocks. Third, thorough, dark surface provided by the cobalt andnickel in the surface portion 2 is one which has much less aflinity formercury than a plain nickel surface. Liquid mercury has little tendencyto amalgamate with this surface portion through the small openings incoating 3 and thus does not destroy the bond between the surface portion2 and coating 3. It has been found that a roughened, plain nickelsurface does not prevent flaking of the emissive coating in the presenceof mercury and that the action of the mercury in wetting the nickelsurface causes an extremely great amount of oxide coating flaking. Onthe other hand, it has been found that a cathode having a surfaceportion 2 in accordance with my invention does not flake because thesurface portion 2 does not wet with mercury. According to an importantaspect of my invention, I provide a convenient, economical andsuccessful process for providing the flaking inhibited surface portion 2on a nickel base member and the following is an example of the processsteps that may be employed.

A powdered mixture of cobalt and nickel compounds, which are readilyreducible to metallic cobalt and nickel, is suspended in a volatilesolution of a binder which com' pletely decomposes into volatileproducts upon heating. For example, such compounds may be oxides,hydroxides, or carbonates of cobalt and nickel; a suitable binder iscellulose nitrate or a polyvinyl resin; and a suitable volatile solventis acetate or amyl acetate. This solution is applied to the surface ofbase member 1 in any suitable manner, both painting with a brush andspraying being satisfactory. The cathode is next heated in a reducingatmosphere to vaporize the binder, reduce the compounds to metalliccobalt and nickel, and to at least partially alloy the resultantmetallic cobalt and nickel. The surface portion 2 is then complete. Theproportion of the cobalt and nickel compounds in the mixture is sochosen that at least 3 percent cobalt, by weight, is provided in thesurface portion 2 and, if desired, up to percent of the cobalt compoundin the mixture may be employed, but percentages in the order of ten (10)percent are high enough in most cases. The emissive coating 3 may nextbe applied to surface portion 2 in a conventional manner. A powderedcarbonate compound of an alkaline earth metal, or a powdered mixture ofcarbonate compounds of more than one alkaline earth metal, suspended ina volatile solvent containing a binder completely vaporizable by heat,is sprayed or brushed on to the surface portion 2. Then the coating isheated in a vacuum to vaporize the binder and change the carbonatecompounds to oxides of alkaline earth metals, the resultant gaseousproducts being drawn off by a suitable means. The flaking inhibitedcathode is then ready for use.

In a preferred specific process for treating the nickel base member toprovide the flaking inhibited surface portion 2 prior to the applicationof emissive coating 3 thereon, I employ a powdered mixture of cobalticand nickelic oxides suspended in a solution of suitable binder such asnitrocellulose or a polyvinyl resin binder and a volatile carrier suchas acetone or amyl acetate and brush the solution on to the base member.The cathode is then fired at 1000 C. for 30 minutes in a reducingatmosphere, such as hydrogen or cracked ammonia gas whereupon the bindervolatilizes, the oxides are reduced to cobalt and nickel, and the cobaltand nickel are in turn at least partially alloyed. The emissive coatingis applied as described above.

As will be understood by those skilled in the art, the heating times andtemperatures. may be varied considerably with the lower temperaturesbeing accompanied by longer heating times, for example, withtemperatures as high as 1075 C. to 1100 C. heating times in the range of3 minutes to 1 minute may be employed while temperatures below 975require an hour or more.

While the present invention has been described by reference to specificillustrations and examples thereof, it will be understood that numerousmodifications may be made by those skilled in the art without actuallydeparting from the invention. I, therefore, aim in the appended claimsto cover all such equivalent variations as come within the scope of theforegoing disclosure.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. For use in electron discharge devices, the combination consisting ofa nickel base member having a surface portion thereon containing atleast three percent cobalt for minimizing flaking of an emissive coatingto be applied thereto and a remainder of substantially pure nickel.

2. For use in electron discharge devices, a cathode consisting of anickel base member, a surface portion on said base member composed of atleast three percent cobalt for minimizing flaking of an emissive coatingapplied thereto and the remainder of said surface portion ofsubstantially pure nickel, and a coating of an electron emissivesubstance on said surface portion.

3. An indirectly heated cathode for use in a mercury vapor dischargedevice consisting of a nickel base member having only the surfaceportion thereof alloyed with cobalt, the cobalt comprising in excess ofthree percent by weight of the surface portion, for minimizing flakingoil? of an emissive coating applied thereto, and a coating of anelectron emissive material on the surface portion thereof.

4. The process of inhibiting flaking of an electron emissive oxidecoating from a nickel cathode base member which comprises the steps ofapplying to the surface of said base member a powdered mixture of acobalt com- ;pound and a nickel compound, said compounds being reducibleto metallic cobalt and nickel respectively, and heating said cathodebase member and applied powders in a reducing atmosphere to atemperature in the order of 1000 C. to reduce said compounds to metalliccobalt and nickel, and to at least partially alloy the metallic cobaltand nickel thus formed with each other and the surface of said basemember.

5. The process of a flaking inhibiting treatment of a nickel cathodeprior to the application thereon of an electron emissive material whichcomprises the steps of applying to the surface of said nickel cathode amixture of cobalt oxide and nickel oxide in a binder which completelydecomposes into volatile products upon heating and a volatile solventfor said binder; and heating said cathode in a reducing atmosphere todecompose and volatilize said binder, and to reduce said cobalt oxideand said nickel oxide to metals and at least partially alloy and fusesaid metals with each other and said surface of said cathode.

6. The process of treating a nickel cathode prior to the applicationthereon of an emissive oxide material which comprises the steps ofapplying to the surface of said cathode a powdered mixture of at leastthree percent cobalt oxide and the remaining percentage of a powderednickel oxide by weight suspended in a binder which completely decomposesinto volatile products upon heating and a volatile solvent for saidbinder; and heating said cathode in a reducing atmosphere to decomposeand volatilize said binder, to reduce said cobalt oxide and said nickeloxide to cobalt and nickel metals and at least partially alloy and fusesaid metals with each other and the surface of said cathode.

7. The process of treating a nickel cathode prior to the applicationthereon of an electron emissive oxide material which comprises the stepsof applying a coating to the surface of said cathode of a powderedmixture including at least three percent by weight of cobalt oxide andthe remaining percentage by weight of nickel oxide suspended in a binderwhich completely decomposes into volatile products upon heating to atemperature less than 1000 C. and a volatile solvent for said binder;and heating said cathode to at least 1000 C. in a reducing atmosphere todecompose and volatilize said binder, to reduce said cobalt oxide andsaid nickel oxide to metallic cobalt and nickel respectively and to atleast partially alloy and fuse said metallic cobalt and said nickel witheach other and the surface portion of said cathode.

8. The process of making a cathode for use in electron discharge deviceshaving mercury vapor therein which comprises the steps of initiallyapplying to the surface of a piece of nickel a powdered mixtureconsisting of at least three percent by weight of powdered cobalt oxideand the remaining percentage of nickel oxide, said mixture beingsuspended in a first binder which completely decomposes into volatileproducts upon heating and a volatile solvent for said first binder;secondly, heating said nickel piece in a reducing atmosphere todecompose and volatilize said first binder, to reduce said cobalt oxideand said nickel oxide to metallic cobalt and nickel, respectively, andto at least partially alloy and fuse said metallic cobalt and nickelwith each other and the surface portion of said piece of nickel;thirdly, applying a powdered carbonate of at least one alkaline earthmetal element sus pended in a second binder which completely decomposesinto volatile products upon heating and a volatile solvent for saidsecond binder; and fourthly, heating said cathode to decompose andvolatilize said second binder and to reduce said alkaline earth metalcarbonate to an alkaline earth oxide, whereby the resultant alkalineearth metal oxide is adhered to the nickel cobalt alloy on the surfaceof said piece of nickel.

9. For use in an electric discharge device, a member consisting of abase portion of at least commercially pure nickel and an alloy surfaceportion containing approximately ten percent cobalt and a remainder ofsubstantially pure nickel only, said surface portion being adapted forminimizing flaking of an emissive coating to be applied to said member.

' 10. For use in an electric discharge device, a cathode comprising abase member consisting of a base portion of at least commercially purenickel and an alloy surface portion containing approximately ten percentcobalt and a remainder of substantially pure nickel only, and a coatingof an emissive substance held firmly on said base member by said surfaceportion.

11. For use in electric discharge devices, a metallic cathode basemember having a dendritic surface portion comprising an admixture ofcobalt and substantially pure nickel only, said dendritic surfaceportion being adapted for minimizing flaking of an electron emissivesubstance to be applied thereto.

12. For use in electric discharge devices, the combination of a basemember having a dendritic surface portion comprising an admixture ofcobalt and substantially pure nickel only, and a coating of an electronemissive substance firmly held on said base member by said dendriticsurface.

13. For use in electric discharge devices, the combination of asubstantially pure nickel base member, said base member having adendritic surface portion fused and alloyed thereto, said dendriticsurface portion comprising an admixture of at least three percent cobaltand a remainder of substantially pure nickel only and being adapted forminimizing flaking of an emissive coating to be applied thereto.

14. For use in electric discharge devices, the combination of asubstantially pure nickel base member, said base member having adendritic surface portion fused and alloyed thereto, said surfaceportion comprising an admixture of at least three percent cobalt and aremainder of substantially pure nickel only, and a coating of electronemissive substance firmly held on said base member by said dendriticsurface.

15. The process of producing a nickel cathode base member adapted forinhibiting flaking of an electron emissive material coating appliedthereto which comprises the steps of applying to the surface of saidmember an admixture of compounds which are reducible to metallic cobaltand nickel, reducing said compounds to metallic cobalt and nickel, andat least partially alloying and fusing the resultant metallic cobalt andnickel with each other and the surface portion of said base member.

References Cited in the file of this patent UNITED STATES PATENTS1,553,394 Pilling Sept. 15, 1925 1,823,984 Nicolson Sept. 22, 19312,049,372 Hamada et al July 28, 1936 2,072,575 Acker et al Mar. 2, 19372,072,576 Acker et al Mar. 2, 1937 2,122,860 Gorlich July 5, 19382,140,367 Lowry Dec. 13, 1938 2,154,633 Mitchell Apr. 18, 1939 2,176,597Sweeney et al Oct. 17, 1939 2,192,491 Widell Mar. 5, 1940

4. THE PROCESS OF INHIBITING FLAKING OF AN ELECTRON EMISSIVE OXIDECOATING FROM A NICKEL CATHODE BASE MEMBER WHICH COMPRISES THE STEPS OFAPPLYING TO THE SURFACE OF SAID BASE MEMBER A POWDERED MIXTURE OF ACOBALT COMPOUND AND A NICKEL COMPOUND, SAID COMPOUNDS BEING REDUCIBLE TOMETALLIC COBALT AND NICKEL RESPECTIVELY, AND HEATING SAID CATHODE BASEMEMBER AND APPLIED POWDERS IN A REDUCING ATMOSPHERE TO A TEMPERATURE INTHE ORDER OF 1000* C. TO REDUCE SAID COMPOUNDS TO METALLIC COBALT ANDNICKEL, AND TO AT LEAST PARTIALLY ALLOY THE METALLIC COBALT AND NICKELTHUS FORMED WITH EACH OTHER AND THE SURFACE OF SAID BASE MEMBER.